app-admin/stackdriver: parse json container logs

This change modifies the cos specific fluentd config used
by the logging agent to automatically parse container
logs in json, while preserving text log functionality.

See b/137508822#comment19 for an example of Before and
After behavior of this change.

BUG=b/137508822
TEST=presubmit, validated behavior on VM
RELEASE_NOTE=Added json parsing for container logs.

Change-Id: If4416bdbbf278a7858007a61d14a93ed456c4709
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/10520
Reviewed-by: Roy Yang <royyang@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
2 files changed